1) Field of the Invention
The present invention relates generally to a temperature regulated cooling system for medium to high average power heat loads, and particularly to a cooling system which utilizes a phase change material to cool these heat loads.
2) Description of Related Art
U.S. Pat. Nos. 6,307,871; 6,351,478; 6,570,895 by Heberle, disclose heat sink assemblies that include heat exchanges with extending surfaces in intimate contact with phase change material.
U.S. Pat. No. 6,354,370 by Miller et al, discloses an open loop liquid spray phase-change cooling system for a laser.
U.S. Pat. No. 6,571,569 by Rini et al, discloses a method and apparatus wherein the coolant to which the heat is transferred can be sprayed onto a surface which is in thermal contact with the heat source, such that the coolant sprayed onto the surface in thermal contact with the heat absorbs heat from the surface and carries the absorbed heat away as the coolant leaves the surface.
Continuous operation of medium to high average power lasers has spurred the development of a class of cooling systems utilizing heat exchangers, water flow loops, and refrigeration units. Characteristically, such cooling systems can add significantly to the overall laser systems' mass, size, and average power consumption. The need to provide cooling to a class of high power lasers that operate only intermittently (“burst mode”) but require minimal mass, size and power consumption has sparked development of alternate cooling systems. Such systems have recognized the benefits of uncoupling the heat removal stage (from the heat load), from the heat elimination stage (from the heat sink). Many of these large lasers are pumped by banks of laser diodes. These laser diode banks must not only be cooled but must be thermally maintained to within about a degree (C) of their optimum operating temperature.
Besides lasers operated in burst mode, other applications exist wherein significant heats are generated when a device is operated intermittently (burst mode), and space/size constraints restrict cooling options. Such non-laser systems include but are not limited to heat loads from fighter plane avionics (radar, targeting and firing electronics, communications jamming systems, etc.). Thermal loads from fighter plane avionics are currently transferred to the craft's fuel, limiting sortie times, or to system to air heat exchangers which negatively impact combat flight characteristics in the form of air drag.
U.S. Pat. No. 5,526,372 by Albrecht et al discloses a method for operating a laser, wherein the waste heat is stored in the lasing medium itself. This approach utilized the inherent heat capacity characteristics of the medium (heat energy stored equals specific heat times the change in temperature).
Cooling systems that utilize phase change materials take advantage of the latent heat able to be absorbed when the material changes phase. Phase change materials provide a much higher thermal energy sink per unit mass and per unit volume of material than the simple heat capacity approach of U.S. Pat. No. 5,526,372 and permits the cooling of a wider range of laser media such as laser diodes and gas media. Since the phase change occurs at a constant temperature, some systems take advantage of this to maintain an interface at the phase change temperature. One downside is that one is limited to the phase change temperature for the specific phase change material, requiring a different phase change material or use of multiple phase change materials if different temperatures are desired. Another serious disadvantage of such an approach is that for high heat transfers with the liquid near the heat of fusion temperature, by definition, the temperature differential between the liquid and the solid phase heat sink is small. Since heat transfer is directly proportional to the temperature differential, on a per unit area basis, this would be a low performance heat exchanger. For high heat loads, such a device would have to be very large and bulky to offer the needed surface area. Also, in systems wherein the heat is transferred via fluid flowing through tubes embedded in the phase change material, the temperature of the solid phase change material surrounding the tubes initially increases (sensible heating) until the phase change temperature is reached. As the phase change material changes from solid to liquid, the vicinity surrounding the tubes begins to be filled with liquid phase change material. At some point the temperature of the liquid phase change material begins to rise due to sensible heating. In other words, over time the operating temperature will rise due to an inherent increase in the thermal impedance of the heat exchanger. Thus, it would be desirable to have a system that mitigates these changes in temperature.